diff --git a/telegram-client.js b/telegram-client.js
index 89c4c1d..bfccc7a 100644
--- a/telegram-client.js
+++ b/telegram-client.js
@@ -918,6 +918,7 @@ class TelegramClient {
         await this._verifyIdentity(authenticatedUser);
       }
 
+      this._restrictSessionFileMode();
       console.log(hasExistingSession ? 'Existing session is valid.' : 'Logged in successfully!');
       return true;
     } catch (error) {
@@ -946,6 +947,16 @@ class TelegramClient {
     }
   }
 
+  // The session file is the account credential, so it gets the same 0600 the store
+  // already gives config.json and account.json. mtcute writes it with the default umask.
+  _restrictSessionFileMode() {
+    try {
+      fs.chmodSync(this.sessionPath, 0o600);
+    } catch {
+      // A missing or foreign-owned session file is not a reason to fail the login
+    }
+  }
+
   async ensureLogin() {
     if (!(await this._isAuthorized())) {
       throw new Error('Not logged in to Telegram. Please restart the server.');
diff --git a/tests/session-file-mode.test.js b/tests/session-file-mode.test.js
new file mode 100644
index 0000000..224413e
--- /dev/null
+++ b/tests/session-file-mode.test.js
@@ -0,0 +1,39 @@
+import fs from 'node:fs';
+import os from 'node:os';
+import path from 'node:path';
+import { afterEach, describe, expect, it } from 'vitest';
+
+import TelegramClient from '../telegram-client.js';
+
+const dirs = [];
+afterEach(() => {
+  while (dirs.length) fs.rmSync(dirs.pop(), { recursive: true, force: true });
+});
+
+const withSessionFile = (mode) => {
+  const dir = fs.mkdtempSync(path.join(os.tmpdir(), 'tgcli-session-'));
+  dirs.push(dir);
+  const sessionPath = path.join(dir, 'session.json');
+  fs.writeFileSync(sessionPath, '{}', { mode });
+  fs.chmodSync(sessionPath, mode);
+  const client = Object.create(TelegramClient.prototype);
+  client.sessionPath = sessionPath;
+  return { client, sessionPath };
+};
+
+describe('session file permissions', () => {
+  it('narrows a world-readable session file to 0600', () => {
+    const { client, sessionPath } = withSessionFile(0o644);
+
+    client._restrictSessionFileMode();
+
+    expect(fs.statSync(sessionPath).mode & 0o777).toBe(0o600);
+  });
+
+  it('does not throw when the session file is absent', () => {
+    const client = Object.create(TelegramClient.prototype);
+    client.sessionPath = path.join(os.tmpdir(), 'tgcli-does-not-exist', 'session.json');
+
+    expect(() => client._restrictSessionFileMode()).not.toThrow();
+  });
+});